(1) Field of the Invention
The present invention relates generally to the removal of hydrogen sulfide from a gas stream utilizing an ammonium hydroxide aqueous solution to absorb the hydrogen sulfide and to the conversion of absorbed hydrogen sulfide to elemental sulfur and the separation of sulfur and the recycling of residual absorbed gases and aqueous liquid to the process.
(2) Description of the Prior Art
The removal of H.sub.2 S from a gas stream is a problem that has long confronted and challenged workers in many diverse industries. One example is in the natural gas industry where the H.sub.2 S content of certain gas streams recovered from natural gas deposits in many areas of the world is often too high for commercial acceptance. Another example is in the manufactured gas industry or the coke-making industry where coal gas containing unacceptable amounts of H.sub.2 S is commonly produced by the destructive distillation of bituminous coal having a high sulfur content. Yet another example is found in the manufacture of water gas or synthesis gas where it is not unusual to produce gas stream containing H.sub.2 S by passing steam over a bed of incandescent coke or coal containing a minor amount of sulfur.
More frequently, this problem is encountered in the petroleum refining industry because the principal raw material used, crude oil, typically contains a minor amount of sulfur--principally in the form of organic sulfur compounds. During the course of the many processes to which the crude oil or fractions thereof are subjected, one or more gas streams containing H.sub.2 S are quite commonly produced. For example, in many cases one of the produce streams from a hydrocarbon conversion process is a gas stream containing H.sub.2 S in admixture with hydrogen and/or with light, normally gaseous hydrocarbons--mainly, C.sub.1 -C.sub.3. As is well known in the art, the presence of H.sub.2 S in these refinery gas streams can cause a number of detrimental problems in subsequent processing steps such as corrosion of process equipment, deterioration and deactivation of catalysts, undesired side reactions, increase in gas compressor capacity, etc.
Regardless of the source of the gas stream containing H.sub.2 S, the problem of removing H.sub.2 S therefrom has been solved in a number of different ways which generally involve one or more of the following techniques: selective absorption with a wide variety of absorbents, adsorption by a suitable adsorbent, selective reaction with a reagent which produces an easily separable product, etc. The details of these techniques are well known to those skilled in the art. One old and well-known solution to this H.sub.2 S removal problem involves scrubbing the gas stream with an ammoniacal aqueous solution. For example, in Germany the Perox process, which uses ammonia scrubbing, has been widely used for coal gas purification. Despite the considerable amount of effort that has been devoted to developing an acceptable solution to this problem, the use of ammoniacal scrubbing has not been universally accepted in the gas treating art as the preferred method for removing H.sub.2 S from a gas stream primarily because of a number of operational difficulties associated with its implementation.
One difficulty involves the relatively high partial pressure of ammonia at preferred scrubbing temperatures which generally requires that the scrubbing step be conducted with a relatively dilute ammonia solution or under relatively high pressure. The use of a dilute scrubbing solution in turn quite commonly forces the addition of a separate water wash step after the ammonia scrubbing step in order to remove ammonia from the treated gas stream. In addition, the use of dilute scrubbing solutions typically increases substantially the regeneration costs where the regeneration step is conducted at a considerably higher temperature than the scrubbing step, although some of this heat load can be recovered by a suitable heat exchanging procedure.
Another difficulty is associated with the regeneration of the rich absorbent solution withdrawn from the H.sub.2 S-scrubbing step. In order to minimize the requirements of the scrubbing step for water and ammonia, it is necessary to remove sulfide from this rich absorbent. Several regeneration procedures have been proposed but they typically have involved the use of absorbent-soluble catalysts such as hydroquinone and have had problems such as contamination of the sulfur product with the catalyst, excessive formation of the undesired by-products such as ammonium sulfate and thiosulfate and loss of scrubbing solution and catalyst during the periodic purges that are generally required to remove side products from the system.
Other difficulties have been associated with the recovery of the elemental sulfur from the regeneration step where in some processes it has been customary to form a froth of sulfur in the absorbent regeneration vessel which then must be skinned off and filtered. In short, it is clear that there are a significant number of technical problems associated with the prior art methods for removing H.sub.2 S from a gas stream by the method of scrubbing with an ammoniacal solution.
Representative continuous processes for removing hydrogen sulfide from a gas stream are those disclosed in U.S. Pat. Nos. 3,715,426 and 3,728,440 to Hamblin. In the processes disclosed in these patents, thermodecomposition of sulfur containing anions to produce sulfur is not used. A catalytic oxidation step is used. An improved process for removing hydrogen sulfide from coke oven gases is disclosed in U.S. Pat. No. 4,342,731 and U.S. Pat. 4,518,572 to Ritter and U.S. Pat. No. 3,249,522 and U.S. Pat. No. 3,409,520 to Bolmer. In the Ritter patents, coke oven gas is washed free of hydrogen sulfide but the sulfur in the hydrogen sulfide is apparently never recovered as elemental sulfur. In the Bolmer patents, hydrogen sulfide is removed from a hydrogen sulfide-hydrocarbon gas mixture utilizing an electrolytic cell containing a porous electrode or a fuel cell containing a porous electrode. The anolyte of the cell can comprise ammonium hydroxide.